In display assembly, bonding a touch panel or cover glass to a dimensional distortion sensitive substrate such as some liquid crystal displays (LCDs) or active matrix organic light emitting diode displays (AMOLED) by means of an optically clear adhesive can be challenging. Newer display designs are calling for thinner and lighter weight components that also have an immediate impact on the design of the actual display panel like the LCD or AMOLED. While thinner glasses that are less fragile are rapidly being adapted by the industry as thin LCD glass or a thin glass layer in an AMOLED, it is also easier to dimensionally deform or distort these thin glass layers, which can lead to unacceptable optical distortions (i.e. Mura) in the final display assembly. For example, if the LCD glass in an LCD panel is locally deformed by as little as a few microns, the actual cell gap filled with liquid crystalline material may also become non-uniform. This may result in varying optical densities that may become visible to the eye as a lighter or darker spot or band in the display. In some cases, the unevenness of the gap between the active display assembly components (i.e. LCD or AMOLED and touch panel or cover glass, or even touch panel to cover glass) can also result in optical distortion because the optical path length is no longer uniform.
Bonding a touch panel or display panel (such as a LCD panel) to a three-dimensional (3-D) cover glass by means of an optically clear adhesive can also be challenging. Indeed, newer designs use cover glasses having a thick (approaching 50 micrometers) ink step around the perimeter or frame of the cover glass, generating a substrate that is no longer flat but has a third dimension to it (i.e. the OCA has to conform to significant differences in the z-dimension of the cover lens substrate). The region encompassed by the ink step is often referred to as a gap. Some pressure sensitive adhesives (PSA), including optically clear adhesives (OCA), may not be compliant enough to conform over the ink area and thus do not completely fill the gap or completely wet the surface of the corresponding viewing area of the display.
In addition to the large ink step, other 3-D features that may require good adhesive wetting of any of the display components include things like the presence of a flex connector, slight curvature of the components, thicker ITO patterns, presence of raised integrated circuits on a touch panel and the like.
Typical PSAs are cross-linked, minimizing their ability to flow. Thus, they may be substantially incompressible, forcing the thinner glass panels to deform during and after bonding to the other display assembly components. Likewise, this lack of flow may result in uneven gaps between display panels, such as a touch panel bonded to a cover glass. In contrast, liquid optically clear adhesives (LOCAs) flow very well and could be considered very compliant in their uncured state. While LOCAs can successfully fill the gap, they may require costly dispensing equipment. LOCAs may also require careful management of the gap setting between the display assembly components (i.e. between cover glass and touch (display) panel, touch panel and LCD, cover glass and AMOLED, etc.) and may require extra cleaning processes to control adhesive overflow. In addition, curing shrinkage can lead to local stresses and deformation of distortion sensitive components, again leading to optical distortions in the display. Once cured, the LOCAs are commonly cross-linked, which can lock in any stresses or distortions generated during curing and assembly.
Mobile handheld (MHH) manufacturers and their display component makers are increasing the ink height (thickness) around the frame of cover glass and decreasing the total thickness and weight of the display to enhance the appearance and decorative features of the display device. Currently, a typical ink step is approximately 5 to 13 micrometers thick and glass components used as part of this display component can be on the order of 0.5 to 1 mm thick. Emerging display components, however, will have ink steps roughly about 50 micrometers thick or more. At the same time, MHH manufacturers would like to make the device as thin as possible, so very thick optically clear adhesive layers are typically not desirable. A conventional approach is to increase the thickness of the adhesive in order to avoid an air gap within the frame. This air gap can result from incomplete wetting of the adhesive in the gap region during initial assembly and/or it may result from excessive residual stress in the adhesive, causing it to slowly lift and detach from the substrate upon relief of the lamination pressure or during durability testing of the display. Likewise, electronic components on the glass may have thicknesses on the order of 10 to 100 μm, making wetting of these features challenging. Lenses may also be warped and curved, making the gap between the lens and the second substrate variable, often on the order of 50 to 100 μm. In some cases, the lenses may intentionally be fabricated with a controlled curvature, creating a gap between the lens and for example, the LCD which is no longer uniform, but in essence has a 3-D feature to it.
Emerging display components also utilize glass layers that are less than 0.5 mm thick. These thin glass layers are more readily deformable and the choice of optical adhesives and the type of assembly process used to put the different layers together becomes very critical. For example, high modulus and highly elastic adhesives may be laminated to a readily distortable or fragile glass panel, possibly causing the glass to become less planar or to fracture. In the case of an AMOLED display panel, even a very small fracture of the glass compromises the barrier properties that are essential for the device to operate. In the case of an LCD, a local loss in planarity of the glass can cause the liquid crystal filled cell gap to change. For example, if the optically clear adhesive cannot relieve the lamination stress quickly or it cannot properly accommodate the difference in thickness between an LCD and for example, a thick ink border printed cover lens, the easy to deform LCD glass may show a local waviness on the order of several microns. To have an optical display assembly that is free of distortion, the planarity of the glass components and uniformity of the gap between panels (for example touch glass to cover lens) should ideally be maintained within about 5 microns or less and particularly 2 microns or less. This planarity or gap uniformity becomes most critical if the micron-size variation happens within a short distance, such as a few centimeters or less.
U.S. Patent Application Publication No. US2009/0029100A1 describes a method for producing optical grade lamination of two rigid substrates using tacky hot melt adhesive films. The substrates described are flat and are not three-dimensional. In addition, the process requires an excess of heat activated material to be used so it can be squeezed out from between the substrates and take the trapped air bubbles with it. The method also requires that non-uniform pressure be applied during the application to allow for bubbles to be removed. The use of excess material and application of non-uniform pressure may make assembly with distortion sensitive or fragile substrates challenging.